No idea what 'naff' means, but you could always write a mod (or pay someone to write it) that would offer that option. The fact that it hasn't been written by now, despite a certain amount of interest, suggests to me that it might be more difficult than it seems. It will be possible in the future version 2.0.
In the meantime, if you receive complaints from women who strongly prefer Mrs. to Ms., you might gently offer the words of Judith Martin, known here in the US as 'Miss Manners' through her syndicated newspaper column on etiquette, in response to an unmarried reader who seemed quite angry at being addressed on invitations as 'Miss' despite her being in her 30's:
"Funny— gentlemen just have “Mr.,” and yet most of them manage to open their mail without carrying on about how the envelope makes them feel."
I'd also suggest that someone who's willing to make a big deal about something so trivial that early in your business relationship may be a 'customer' best lost - you can bet she'll find other meaningless ways to waste your time later on.